Fix runtimeType for strings, integers and double: return their interface type.

runtime/lib/integers.dart

Index: runtime/lib/integers.dart
===================================================================
--- runtime/lib/integers.dart	(revision 28266)
+++ runtime/lib/integers.dart	(working copy)
@@ -9,6 +9,9 @@
     throw new UnsupportedError(
         "_IntegerImplementation can only be allocated by the VM");
   }
+
+  Type get runtimeType => int;
+
   num operator +(num other) {
     return other._addFromInteger(this);
   }